Method and apparatus for concurrent atrio-ventricular anti-tachycardia pacing

ABSTRACT

An implantable medical device delivers anti-tachyarrhythmia therapies including anti-tachycardia pacing (ATP). If a detected tachyarrhythmia is classified as a type suitable for treatment using ATP, the implantable medical device selects one of an atrial ATP (A-ATP) mode, a ventricular ATP (V-ATP) mode, and a concurrent atrio-ventricular ATP (concurrent AV-ATP) mode according to the characteristics of the detected tachyarrhythmia. The concurrent ATP mode is an ATP mode during which the atrial pacing pulses and the ventricular pacing pulses are delivered concurrently. In one embodiment, the concurrent AV-ATP mode includes a synchronized atrio-ventricular ATP (synchronized AV-ATP) mode during which atrial and ventricular pacing pulses are delivered synchronously and an independent atrio-ventricular ATP (independent AV-ATP) mode during which atrial and ventricular pacing pulses are delivered concurrently but timed independently.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. 119(e) of U.S.Provisional Patent Application Ser. No. 60/978,972, entitled “Method andApparatus for Concurrent Atrio-Ventricular Anti-Tachycardia Pacing,”filed on Oct. 10, 2007, which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

This document relates generally to cardiac rhythm management (CRM)systems and particularly to a system providing for anti-tachycardiapacing (ATP), including concurrent delivery of ATP to both atrial andventricular chambers.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidheart rate. Tachyarrhythmia generally includes supraventriculartachyarrhythmia and ventricular tachyarrhythmia. Fibrillation is a formof tachyarrhythmia further characterized by an irregular heart rhythm.In a normal heart, the sinoatrial (SA) node, the heart's predominantnatural pacemaker, generates electrical impulses, called actionpotentials, that propagate through an electrical conduction system tothe atria and then to the ventricles of the heart to excite themyocardial tissues. The atria and ventricles contract in the normalatrio-ventricular sequence and synchrony to result in efficientblood-pumping functions indicated by a normal hemodynamic performance.Ventricular tachyarrhythmia occurs when the electrical impulsespropagate along a pathologically formed self-sustaining conductive loopwithin the ventricles or when a biologic pacemaker (focus) in aventricle usurps control of the heart rate from the SA node. When theatria and the ventricles become dissociated during ventriculartachyarrhythmia, the ventricles may contract before they are properlyfilled with blood, resulting in diminished blood flow throughout thebody. This condition becomes life-threatening when the brain is deprivedof sufficient oxygen supply. Ventricular fibrillation (VF), inparticular, stops blood flow within seconds and, if not timely andeffectively treated, causes immediate death. In very few instances aheart recovers from VF without treatment.

Ventricular cardioversion and defibrillation are used to terminate mostventricular tachyarrhythmias, including ventricular tachycardia (VT),and VF. An implantable cardioverter/defibrillator (ICD) is a CRM devicethat delivers cardioversion/defibrillation pulses, each being anelectric shock, to terminate a detected tachyarrhythmia episode bydepolarizing the entire myocardium simultaneously and rendering itrefractory.

Another type of electrical therapy for tachyarrhythmia is ATP, includingatrial ATP for treating atrial tachyarrhythmia and ventricular ATP fortreating ventricular tachyarrhythmia. In ATP, the heart is competitivelypaced in an effort to interrupt the reentrant loop causing thetachyarrhythmia. In an ICD that includes ATP andcardioversion/defibrillation capabilities, the efficacy of eachavailable anti-tachyarrhythmia therapy depends on the type and origin ofthe tachyarrhythmia. For example, a ventricular anti-tachycardia pacingtherapy is generally ineffective in terminating an atrialtachyarrhythmia. Additionally, the delivery of eachcardioversion/defibrillation pulse consumes a considerable amount ofpower and results in patient discomfort owing to the high voltage of theshock pulses. If delivered during the atrial vulnerable period, acardioversion/defibrillation pulse may also cause atrial fibrillation.Therefore, for therapy efficacy, device longevity, and patientsatisfaction, it is desirable for an ICD to use ATP to terminate adetected tachyarrhythmia wherever possible. For this and other reasons,there is a need for expanding the capability and improving theeffectiveness of ATP in terminating tachyarrhythmias.

SUMMARY

An implantable medical device delivers anti-tachyarrhythmia therapiesincluding anti-tachycardia pacing (ATP). If a detected tachyarrhythmiais classified as a type suitable for treatment using ATP, theimplantable medical device selects one of an atrial ATP (A-ATP) mode, aventricular ATP (V-ATP) mode, and a concurrent atrio-ventricular ATP(concurrent AV-ATP) mode according to the characteristics of thedetected tachyarrhythmia. The concurrent ATP mode is an ATP mode duringwhich the atrial pacing pulses and the ventricular pacing pulses aredelivered concurrently. In one embodiment the concurrent AV-ATP modeincludes a synchronized atrio-ventricular ATP (synchronized AV-ATP) modeduring which atrial and ventricular pacing pulses are deliveredsynchronously and an independent atrio-ventricular ATP (independentAV-ATP) mode during which atrial and ventricular pacing pulses aredelivered concurrently but timed independently.

In one embodiment, an implantable cardiac rhythm management (CRM) deviceincludes a pacing circuit, a tachyarrhythmia detection andclassification circuit, and an ATP control circuit. The pacing circuitdelivers atrial pacing pulses and ventricular pacing pulses. Thetachyarrhythmia detection and classification circuit includes a cardiacsensing circuit, a rate detector, a tachyarrhythmia detector, and atachyarrhythmia classifier. The cardiac sensing circuit senses cardiacsignals indicative of atrial depolarizations and ventriculardepolarizations. The rate detector detects an atrial rate and aventricular rate using the cardiac signals. The tachyarrhythmia detectordetects tachyarrhythmia using the ventricular rate and one or moretachyarrhythmia threshold rates. The tachyarrhythmia classifierclassifies the detected tachyarrhythmia. The ATP control circuitcontrols delivery of one or more of the atrial pacing pulses and theventricular pacing pulses according to a selected ATP mode and includesan ATP mode selector. The ATP mode selector selects one of the A-ATPmode, V-ATP mode, and concurrent AV-ATP mode using one or more specifiedATP mode selection criteria and the classification of the detectedtachyarrhythmia.

In one embodiment, a method for operating an implantable CRM device isprovided. Cardiac signals indicative of atrial depolarizations andventricular depolarizations are sensed. An atrial rate and a ventricularrate are detected using the cardiac signals. Tachyarrhythmia is detectedusing the ventricular rate and one or more tachyarrhythmia thresholdrates. The detected tachyarrhythmia is classified. An ATP mode isselected from the A-ATP mode, V-ATP mode, and concurrent AV-ATP modeusing one or more specified ATP mode selection criteria and theclassification of the detected tachyarrhythmia. Delivery of one or moreof atrial pacing pulses and the ventricular pacing pulses are controlledaccording to the selected ATP mode.

This Summary is an overview of some of the teachings of the presentapplication and not intended to be an exclusive or exhaustive treatmentof the present subject matter. Further details about the present subjectmatter are found in the detailed description and appended claims. Otheraspects of the invention will be apparent to persons skilled in the artupon reading and understanding the following detailed description andviewing the drawings that form a part thereof. The scope of the presentinvention is defined by the appended claims and their legal equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate generally, by way of example, variousembodiments discussed in the present document. The drawings are forillustrative purposes only and may not be to scale.

FIG. 1 is an illustration of an embodiment of a CRM system and portionsof the environment in which the CRM system operates.

FIG. 2 is a block diagram illustrating an embodiment of an ICD and alead system of the CRM system.

FIG. 3 is a block diagram illustrating an embodiment of atachyarrhythmia detection and classification circuit of the ICD.

FIG. 4 is a flow chart illustrating a method for classifying detectedtachyarrhythmia.

FIG. 5 is a block diagram illustrating an embodiment of a circuit of apacing controller of the ICD.

FIG. 6 is a flow chart illustrating an embodiment of a method forcontrolling ATP.

FIG. 7 is a flow chart illustrating an embodiment of a method forselecting an ATP mode.

FIG. 8 is a flow chart illustrating another embodiment of a method forselecting an ATP mode.

FIG. 9 is a flow chart illustrating an embodiment of a method for timingATP delivering according to a synchronized AV-ATP mode.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the scope of the presentinvention. The following detailed description provides examples, and thescope of the present invention is defined by the appended claims andtheir legal equivalents.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one. In this document, the term“or” is used to refer to a nonexclusive or, unless otherwise indicated.Furthermore, all publications, patents, and patent documents referred toin this document are incorporated by reference herein in their entirety,as though individually incorporated by reference. In the event ofinconsistent usages between this documents and those documents soincorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

It should be noted that references to “an”, “one”, or “various”embodiments in this document are not necessarily to the same embodiment,and such references contemplate more than one embodiment.

The relationship between a heart rate and a cardiac cycle length (alsoknown as cardiac interval), as used in this document, is therelationship between a frequency and its corresponding period. If aheart rate is given in beats per minute (bpm), its corresponding cardiaccycle length in milliseconds is calculated by dividing 60,000 by theheart rate (where 60,000 is the number of milliseconds in a minute). Anyprocess, such as a comparison, using a heart rate is to be modifiedaccordingly when a cardiac cycle length is used instead. For example, ifa tachyarrhythmia is detected when the ventricular rate exceeds atachyarrhythmia threshold rate, an equivalent process is to detect thetachyarrhythmia when the ventricular cycle length (also known asventricular interval) falls below a tachyarrhythmia threshold interval.The appended claims should be construed to cover such variations.

In this document, a “fast beat” refers to a heart beat having a heartrate that falls into a tachyarrhythmia detection zone, which istypically defined by at least one tachyarrhythmia detection threshold,and a “slow beat” refers to a heart beat having a heart rate that isbelow the tachyarrhythmia detection zone. In other words, a “fast beat”is a heart beat having a tachyarrhythmic heart rate, and a “slow beat”is a heart beat having a heart rate that is not tachyarrhythmic.

In this document, “atrial ATP (A-ATP) mode” refers to an ATP mode duringwhich only atrial pacing pulses are delivered, “ventricular ATP (V-ATP)mode” refers to an ATP mode during which only ventricular pacing pulsesare delivered, and “concurrent atrio-ventricular ATP (concurrent AV-ATP)mode” refers to an ATP mode during which atrial and ventricular pacingpulses are concurrently delivered. The concurrent AV-ATP mode includestwo specific modes of operation: “independent atrio-ventricular ATP(independent AV-ATP) mode” and “synchronized atrio-ventricular ATP(AV-ATP) mode”. During the independent AV-ATP mode, atrial andventricular pacing pulses are delivered concurrently but not necessarilysynchronously. During the synchronized AV-ATP mode, atrial andventricular pacing pulses are delivered approximately simultaneously orwith a synchronization offset. In various embodiments, thesynchronization offset is programmable and/or dynamically adjustable.

This document discusses, among other things, an ICD that deliversanti-tachyarrhythmia therapies including ATP. Available ATP operationmodes include A-ATP mode, V-ATP mode, and concurrent AV-ATP mode. Inresponse to a detection of tachyarrhythmia, the ICD classifies thetachyarrhythmia and selects an anti-tachyarrhythmia therapy. If ATP isselected, the ICD further selects one of the ATP operation modesaccording to the classification and various detected characteristics ofthe detected tachyarrhythmia. The concurrent AV-ATP mode is selected,for example, when the detected tachyarrhythmia has characteristicsindicative of an origin in atria, atrio-ventricular node, or ventricularbasal (high-septal) areas (i.e., ventricular regions close to atria) ora reentrant loop that covers at least an atrium and a ventricle. In suchexamples, the detected tachyarrhythmia may be difficult to classify withhigh certainty, but likely to be terminated by pacing on the concurrentAV-ATP mode. In one scenario, AF with short stable periods, fastventricular response, and a waveform marginally correlated to a normalsinus rhythm waveform may be detected as VT, but pacing at the V-ATPmode is unlikely to terminate AF. In another scenario, it may bedifficult to determine whether a 1:1 tachyarrhythmia (in which atrialand ventricular rates are approximately equal) is a VT with retrogradeconduction or a supraventricular tachycardia (SVT) with fast ventricularresponse, so pacing at either the V-ATP mode or the A-ATP mode may notbe effective. Pacing at the concurrent AV-ATP mode in such scenariosimproves the effectiveness of the ATP therapy by bypassing thedifficulty or uncertainty associated with the classification of thedetected tachyarrhythmia. The concurrent mode further includesindependent AV-ATP mode and synchronized AV-ATP mode. The synchronizedAV-ATP mode is selected when a synchronized delivery of atrial andventricular pacing pulses is feasible, such as during the 1:1tachyarrhythmia. The independent AV-ATP mode is selected when asynchronized delivery of atrial and ventricular pacing pulses isdifficult or impossible to control, such as when the atrial rate issubstantially higher than the ventricular rate. During the synchronizedAV-ATP mode, the delivery of the atrial pacing pulses and the deliveryof the ventricular pacing pulses are synchronized. During theindependent AV-ATP mode, the delivery of the atrial pacing pulses andthe delivery of the ventricular pacing pulses are independently timed.

FIG. 1 is an illustration of an embodiment of a CRM system too andportions of the environment in which CRM system 100 operates. CRM system100 includes an ICD 101 that is electrically coupled to a heart 199through leads 105 and 110. An external system 102 communicates with ICD101 via a telemetry link 103.

ICD 101 is an implantable medical device that performs CRM functionsincluding delivery of cardiac pacing and cardioversion/defibrillationtherapies. ICD 101 includes a hermetically sealed can housing anelectronic circuit that senses physiological signals and deliverstherapeutic electrical pulses. The hermetically sealed can may alsofunction as an electrode for sensing and/or pulse delivery purposes. Inone embodiment, as illustrated in FIG. 1, the electronic circuit sensesat least an atrial electrogram and a ventricular electrogram from heart199 and delivers pacing and cardioversion/defibrillation pulses to heart199. Lead 105 is typically a pacing lead that includes a proximal end106 connected to ICD 101 and a distal end 107 placed in the right atrium(RA) of heart 199. A pacing-sensing electrode 108 is located at distalend 107. Another pacing-sensing electrode 109 is located near distal end107. Electrodes 108 and 109 are electronically corrected to ICD 101 viaseparate conductors in lead 105 to allow for sensing of the atrialelectrogram and/or delivery of atrial pacing pulses. Lead 110 istypically a defibrillation lead that includes a proximal end 111connected to ICD 101 and a distal end 112 placed in the right ventricle(RV) of heart 199. A pacing-sensing electrode 113 is located at distalend 112. A defibrillation electrode 114 is located near distal end 112but electrically separated from pacing-sensing electrode 113. Anotherdefibrillation electrode 115 is located at a distance from distal end112 for supraventricular placement. Electrodes 113, 114, and 115 areelectrically connected to ICD 101 via separate conductors in lead 110.Electrode 113 and 114 allow for sensing of the ventricular electrogramand/or delivery of ventricular pacing pulses. Electrodes 114 and 115allow for delivery of ventricular cardioversion/defibrillation pulses.The functions of these electrodes are discussed above by way of exampleand not by way of limitation. Other ways of using these electrodes arepossible as understood by those of skill in the art.

ICD 101 includes an anti-tachyarrhythmia system 120 providing foranti-tachyarrhythmia therapies including ATP therapies in the A-ATPmode, V-ATP mode, or concurrent AV-ATP mode. The A-ATP mode includesdelivery of atrial pacing pulses. The V-ATP mode includes delivery ofventricular pacing pulses. The concurrent AV-ATP mode includesconcurrent delivery of atrial and ventricular pacing pulses. Twospecific concurrent AV-ATP modes are synchronized AV-ATP mode andindependent AV-ATP mode. Synchronized AV-ATP mode includes delivery ofatrial and ventricular pacing pulses in a synchronized manner.Independent AV-ATP mode includes concurrent delivery of atrial andventricular pacing pulses with the delivery of pacing pulses to eachchamber independently timed. In response to a detected tachyarrhythmiaepisode, ICD 101 analyzes the sensed atrial and ventricular electrogramsto classify and characterize the tachyarrhythmia. If the detectedtachyarrhythmia is classified as a type suitable for an ATP therapy, oneof the A-ATP mode, V-ATP mode, and concurrent AV-ATP mode is selectedusing the classification and characteristics of the tachyarrhythmia. Invarious embodiments, the concurrent AV-ATP mode improves the efficacy ofATP when compared to single-chamber ATP modes, especially when thereentrant circuit of the tachyarrhythmia loops in both atrial andventricular chambers or when it is difficult to locate the origin of thetachyarrhythmia with a satisfactory level of certainty. In oneembodiment, one of the synchronized AV-ATP mode and independent AV-ATPmode is selected as the concurrent AV-ATP mode based on whethersynchronization between the atrial pacing pulses and the ventricularpacing pulses is feasible. Anti-tachyarrhythmia system 120 is furtherdiscussed below, with references to FIGS. 2-9.

External system 102 allows for programming of ICD 101 and receivessignals acquired by ICD 101. In one embodiment, external system 102includes a programmer. In another embodiment, external system 102 is apatient management system including an external device in proximity ofICD 101, a remote device in a relatively distant location, and atelecommunication network linking the external device and the remotedevice. The patient management system provides for access to ICD 101from a remote location, such as for monitoring patient status and/oradjusting therapies. In one embodiment, telemetry link 103 is aninductive telemetry link. In an alternative embodiment, telemetry link103 is a far-field radio-frequency telemetry link. Telemetry link 103provides for data transmission from ICD 101 to external system 102. Thismay include, for example, transmitting real-time physiological dataacquired by ICD 101, extracting physiological data acquired by andstored in ICD 101, extracting therapy history data stored in ICD 101,and extracting data indicating an operational status of ICD 101 (e.g.,battery status and lead impedance). Telemetry link 103 also provides fordata transmission from external system 102 to ICD 101. This may include,for example, programming ICD 101 to acquire physiological data,programming ICD 101 to perform at least one self-diagnostic test (suchas for a device operational status), programming ICD 101 to run a signalanalysis algorithm (such as an algorithm implementing a tachyarrhythmiaclassification method discussed in this document), and programming ICD101 to deliver pacing and/or cardioversion/defibrillation therapies.

The circuit of ICD 101, including its various elements discussed in thisdocument, may be implemented using a combination of hardware andsoftware. In various embodiments, each element of ICD 101 discussed inthis document may be implemented using an application-specific circuitconstructed to perform one or more particular functions or ageneral-purpose circuit programmed to perform such function(s). Such ageneral-purpose circuit includes, but is not limited to, amicroprocessor or a portion thereof, a microcontroller or portionsthereof, and a programmable logic circuit or a portion thereof. Forexample, a “comparator” includes, among other things, an electroniccircuit comparator constructed to perform the only function of acomparison between two signals or a portion of a general-purpose circuitdriven by a code instructing that portion of the general-purpose circuitto perform the comparison between the two signals.

FIG. 2 is a block diagram illustrating an embodiment of an ICD 201 and alead system 204. Lead system 204 includes one or more leads such asleads 105 and 110. ICD 201 is a specific embodiment of ICD 101 andincludes a tachyarrhythmia detection and classification circuit 220, atherapy circuit 224, and a therapy controller 226. Tachyarrhythmiadetection and classification circuit 220 detects and classifiestachyarrhythmia episode using at least one or more intrinsic electricalcardiac signals sensed using lead system 204. In one embodiment, inaddition to one or more cardiac signals, tachyarrhythmia detection andclassification circuit 220 uses one or more other physiological signals,such as one or more signals indicative of hemodynamic performance, todetect and classify tachyarrhythmia episode. Therapy circuit 224includes a pacing circuit 228 to deliver pacing pulses to heart 199through lead system 204 and a defibrillation circuit 230 to delivercardioversion/defibrillation pulses to heart 199 through lead system204. Therapy controller 226 includes a pacing controller 232 to controlthe delivery of the pacing pulses, including ATP pulses, and adefibrillation controller 234 to control the delivery of thecardioversion/defibrillation pulses. Therapy controller 226 selects oneor more of pacing and cardioversion/defibrillation therapies based onthe classification of the tachyarrhythmia episode. In one embodiment, anATP therapy is delivered when a detected tachyarrhythmia is classifiedas a type of tachyarrhythmia known to be treatable by the ATP therapy.If the ATP therapy fails to terminate the tachyarrhythmia, acardioversion/defibrillation therapy is delivered.

FIG. 3 is a block diagram illustrating an embodiment of atachyarrhythmia detection and classification circuit 320.Tachyarrhythmia detection and classification circuit 320 is a specificembodiment of tachyarrhythmia detection and classification circuit 220and includes a cardiac sensing circuit 340, a rate detector 342, atachyarrhythmia detector 344, a tachyarrhythmia classifier 346, and anAV association detector 360.

Cardiac sensing circuit 340 senses one or more cardiac signals, such asone or more electrograms, through lead system 204. In one embodiment,cardiac sensing circuit 340 is electrically coupled to heart 199 throughlead system 204 to sense an atrial electrogram and a ventricularelectrogram from the heart. The atrial electrogram includes atrialevents, also known as P waves, each indicative of an atrialdepolarization. The ventricular electrogram includes ventricular events,also known as R waves, each indicative of a ventricular depolarization.

Rate detector 342 detects one or more heart rates from one or morecardiac signals sensed by cardiac sensing circuit 340. In oneembodiment, rate detector 342 detects an atrial rate from the atrialelectrogram and a ventricular rate from the ventricular electrogram. Theatrial rate is the frequency of the atrial events. The ventricular rateis the frequency of the ventricular events. In one embodiment, theatrial and ventricular rates are each expressed in beats per minute(bpm), i.e., number of detected atrial or ventricular depolarizationsper minute.

Tachyarrhythmia detector 344 detects a tachyarrhythmia episode. In oneembodiment, a tachyarrhythmia is detected when the ventricular rateexceeds a predetermined tachyarrhythmia threshold rate. In oneembodiment, tachyarrhythmia detector 344 detects tachyarrhythmia bydetermining whether the ventricular rate is within one of a plurality oftachyarrhythmia rate zones each including a predetermined thresholdrate. In a specific embodiment, the plurality of tachyarrhythmia ratezones includes a VF rate zone with a VF threshold rate programmablebetween 130 and 250 bpm, a fast VT rate zone with a fast VT thresholdrate programmable between 110 and 210 bpm, and a slow VT rate zone witha slow VT threshold rate programmable between 90 and 200 bpm. In anotherembodiment, the tachyarrhythmia is detected using a “zonelesstachyarrhythmia detection” method, as discussed in U.S. patentapplication Ser. No. 11/301,716, “ZONELESS TACHYARRHYTHMIA DETECTIONWITH REAL-TIME RHYTHM MONITORING”, filed on Dec. 13, 2005, assigned toCardiac Pacemakers, Inc., which is incorporated herein by reference inits entirety.

Tachyarrhythmia classifier 346 classifies each tachyarrhythmia detectedby tachyarrhythmia detector 344. Examples of classification oftachyarrhythmia made by tachyarrhythmia classifier 346 includeventricular fibrillation (VF), ventricular tachycardia (VT), andsupraventricular tachycardia (SVT), which includes atrial fibrillation(AF), atrial flutter (AFL), sinus tachycardia (ST), and atrialtachycardia (AT). In one embodiment, a detected tachyarrhythmia isclassified as VF when the ventricular rate falls within the VF ratezone, without further analysis by tachyarrhythmia classifier 346. In theillustrated embodiment, tachyarrhythmia classifier 346 includes a ratecomparator 348, an onset rate analyzer 350, a stability analyzer 352, acorrelation analyzer 354, a correlation threshold adjuster 356, and aconfidence level analyzer 358. Rate comparator 348 compares the atrialrate and the ventricular rate to determine whether the atrial rateexceeds, equals, or is lower than the ventricular rate by apredetermined margin. Onset rate analyzer 350 produces an onset rate ofthe detected tachyarrhythmia and determines whether the detectedtachyarrhythmia has a gradual onset or a sudden onset by comparing theonset rate to one or more threshold onset rates. The onset rate is arate of transition of the ventricular rate from a normal sinus rate to atachyarrhythmic rate when the detected tachyarrhythmia begins. A gradualonset typically indicates a physiological tachyarrhythmia, such as an STcaused by exercise. A sudden onset typically indicates a pathologicaltachyarrhythmia. Stability analyzer 352 produces a rate stabilityparameter indicative of a degree of heart rate variability anddetermines whether the heart rate is stable by comparing the stabilityparameter to a stability threshold. In one embodiment, the stabilityparameter is produced as an average variance of a series of cardiacintervals. In one embodiment, stability analyzer 352 produces aventricular rate stability parameter and an atrial stability parameter.The ventricular rate stability parameter is indicative of a degree ofventricular rate variability. The atrial stability parameter isindicative of a degree of atrial rate variability. Stability analyzer352 determines whether the ventricular rate is stable by comparing theventricular rate stability parameter to a ventricular stabilitythreshold and determines whether the atrial rate is stable by comparingthe atrial stability parameter to an atrial stability threshold. In oneembodiment, stability analyzer 352 compares the ventricular ratestability parameter and the atrial stability parameter to determinewhich of the ventricular rate and the atrial rate is more stable.Correlation analyzer 354 analyzes a correlation between atachyarrhythmic waveform and a template waveform and produces acorrelation coefficient representative of that correlation. Thetachyarrhythmic waveform includes a segment of a cardiac signal sensedduring the detected tachyarrhythmia. The template waveform is recordedduring a known cardiac rhythm such as the normal sinus rhythm (NSR). Oneexample for producing such a correlation coefficient, referred to as afeature correlation coefficient (FCC), is discussed in U.S. Pat. No.6,708,058, “NORMAL CARDIAC RHYTHM TEMPLATE GENERATION SYSTEM ANDMETHOD,” assigned to Cardiac Pacemakers, Inc., which is herebyincorporated in its entirety. In one embodiment, the detectedtachyarrhythmia is considered as “correlated” if a correlationcoefficient exceeds a correlation threshold and as “marginallycorrelated” if the correlation coefficient exceeds a marginalcorrelation threshold that is lower than the correlation threshold.Correlation threshold adjuster 356 allows adjustment of the marginalcorrelation threshold. Tachyarrhythmia classifier 346 classifies thedetected tachyarrhythmia using one or more of the atrial rate,ventricular rate, onset rate, stability parameter, and correlationcoefficient.

Confidence level analyzer 358 determines a confidence level associatedwith a classification of the detected tachyarrhythmia. The confidencelevel indicates a level of certainty in the classification made bytachyarrhythmia classifier 346. In one embodiment, the confidence levelhas a value between 0 and 1. In one embodiment, confidence levelanalyzer 358 determines a confidence level associated with each VTclassification of the detected tachyarrhythmia. In one embodiment, theconfidence level of classifying the detected tachyarrhythmia as VT isdetermined using a probabilistic detection technique. An approximateprobability of classifying a detected tachyarrhythmia as VT isestablished by measuring a multidimensional distance of a VTcharacteristic vector to a composite threshold vector. The distance of acharacteristic variable C_(i) from its threshold T_(i) is denoted by∥C_(i)−T_(i)∥, where ∥.∥ is a generic distance measure. Examples forsuch a distance measure include Euclidian distance, Mahalanobisdistance, and correlation coefficient. The distance measure is thenmapped to a scalar d_(i) (0<d_(i)<1), which represents the confidencelevel for the characteristic variable C_(i) to correctly classify therhythm as VT, i.e., mapping function f: ∥C_(i)−T_(i)∥→d_(i). In oneembodiment, the mapping function f takes the form such thatd_(i)=exp(−1/∥C_(i)−T_(i)∥). In one embodiment, ∥C_(i)−T_(i)∥ is scaledto a range such that d_(i) takes value in full range between 0 and 1,i.e. d_(i)=exp(−1/(a·∥C_(i)−T_(i)∥)), where a is a scaling factor. Forexample, when ∥C_(i)−T_(i)∥ is scaled to between 0.1 and 100, d_(i) hasthe value between 0 and 0.99. In one embodiment, a is determined usingan estimate of a reasonable value range of C_(i) and the thresholdT_(i). For example, if C_(i) is the ventricular rate stability, then asmall stability favors a VT classification. A reasonably estimatedlowest value for C_(i) is 0 ms, so the maximum distance is 20 ms. Thescaling factor a is then set to 5 such that when C_(i) is 0 ms, d_(i) is0.99, which indicates a very high confidence for a VT classification.When the difference between C_(i) and its threshold T_(i) increases, thedistance approaches 1, indicating increased confidence that the VTclassification is correct. When the difference between C_(i) and itsthreshold T_(i) decreases, the distance approaches 0, indicatingdecreased confidence that the VT classification is correct. For allcharacteristic variables i=1, 2, . . . N, the composite distance D isdetermined as the linear combination of the d_(i) (i=1, 2, . . . N),i.e., D=Σα_(i)d_(i), where α_(i) is the weight for C_(i), withconstraint 0<α₁<1 and Σα_(i)=1. In one embodiment, α_(i) is determinedusing a user's experience with the characteristic variables (C_(i)'s).In another embodiment, α_(i) is determined as being inverselyproportional to the variability of C_(i). For example, if C_(i) is thestability value that is calculated as the average of k cardiac cyclelengths, a large variance of C_(i) suggests a low reliability of theaverage C_(i) and hence a smaller α_(i) is assigned. A VT classificationis associated with a relatively low confidence level if the factors usedin the classification process, such as the one or more of the atrialrate, ventricular rate, onset rate, stability parameter, and correlationcoefficient, exceeds their respective thresholds by a relatively lowmargin.

AV association detector 360 detects a stable AV association using theatrial depolarizations and ventricular depolarizations sensed by cardiacsensing circuit 340 over a time interval. The stable AV associationindicates a substantially stable temporal relationship between theatrial depolarizations and ventricular depolarizations sensed over thetime interval. In the illustrated embodiment, AV association detector360 includes a stable AV ratio detector 362 and a stable intervaldetector 364. Stable AV ratio detector 362 detects a substantiallystable AV ratio, which is a substantially stable ratio of atrialdepolarizations to ventricular depolarizations. Stable AV ratio detector362 indicates the detection of the stable AV association when thesubstantially stable AV ratio is detected. Stable interval detector 364detects substantially stable AV or ventriculo-atrial (VA) intervals andindicates the detection of the stable AV association when thesubstantially stable AV or VA intervals are detected. In one embodiment,whether the AV ratio and the AV or VA intervals are stable are eachdetermined using its variability over time. In one embodiment, the AVratio is considered substantially stable when the standard deviation ofthe ratio of the atrial rate to the ventricular rate over consecutivetime windows is less than 10% of an average ratio of the atrial rate tothe ventricular rate. In one embodiment, the AV or VA interval isconsidered substantially stable when the standard deviation of the AV orVA interval is less than 10% of an average AV or VA interval.

In one embodiment, tachyarrhythmia classifier 346 classifies thedetected tachyarrhythmia using a method discussed below with referenceto FIG. 4. The classification of the detected tachyarrhythmia, as wellas the various characteristics such as the atrial rate, ventricularrate, onset rate, stability parameter, correlation coefficient, andstable AV association, are used for selecting a suitableanti-tachyarrhythmia therapy, including an ATP mode if the ATP therapyis to be delivered.

In one embodiment, tachyarrhythmia detector 344 performs a detectionprocess that is initiated by a detection of three consecutive fast beatsfrom the ventricular electrogram. In response to the detection of threeconsecutive fast beats, a tachyarrhythmia detection window is started.The tachyarrhythmia detection window includes ten consecutively detectedheart beats starting with and including the three consecutive fastbeats. If at least eight out of the ten heart beats in thetachyarrhythmia detection window are fast beats (i.e., thetachyarrhythmia detection window is satisfied), a tachyarrhythmiaverification duration is started. Otherwise, the tachyarrhythmiaverification duration is not started.

During the tachyarrhythmia verification duration, a moving verificationwindow of ten consecutively detected heart beats is used to determinewhether the detected tachyarrhythmia sustains. If at least six out ofthe ten heart beats in the verification window are fast beats (i.e., theverification window is satisfied), the detected tachyarrhythmia isconsidered to be sustaining. If this verification window fails to besatisfied at any time during the tachyarrhythmia verification duration,the tachyarrhythmia detection is terminated without delivering ananti-tachyarrhythmia therapy. If the detected tachyarrhythmia episode isdetermined to be sustaining throughout the tachyarrhythmia verificationduration, it is classified by tachyarrhythmia classifier 346 todetermine the necessity and type of an anti-tachyarrhythmia therapy.

If the detected tachyarrhythmia is classified as a type oftachyarrhythmia for which a ventricular cardioversion/defibrillationtherapy is to be delivered, such as a VT episode, the preparation forthe delivery of the ventricular cardioversion/defibrillation shock pulseis started. In one embodiment, an ATP therapy is delivered during thepreparation for the delivery of the ventricularcardioversion/defibrillation shock pulse. After the preparation for thedelivery of the ventricular cardioversion/defibrillation shock pulse iscompleted, a tachyarrhythmia reconfirmation window of three consecutiveheart beats is started, immediately before a scheduled ventricularcardioversion/defibrillation pulse delivery. If at least two out of thethree heart beats in the tachyarrhythmia reconfirmation window are fastbeats (i.e., the tachyarrhythmia reconfirmation window is satisfied),the detected tachyarrhythmia is considered to be still sustaining, andthe ventricular cardioversion/defibrillation pulse is delivered. On theother hand, if the detected tachyarrhythmia is classified as a type oftachyarrhythmia for which a ventricular anti-tachycardia pacing therapyis to be delivered, ventricular anti-tachycardia pacing pulses aredelivered without starting the reconfirmation window for checkingwhether the detected tachyarrhythmia sustains.

If the detected tachyarrhythmia episode is classified as a type oftachyarrhythmia for which no ventricular anti-tachyarrhythmia therapy isneeded, such as an SVT episode, a sustained rate duration (SRD) timewindow may be started, depending on whether it is programmed to beapplied. During the SRD, the ventricular rate is monitored to determinewhether the tachyarrhythmia episode sustains. If the tachyarrhythmiaepisode sustains throughout the SRD, the ventricularanti-tachyarrhythmia therapy is delivered when the SRD expires eventhough the detected tachyarrhythmia episode is classified as an SVTepisode. The tachyarrhythmia episode sustains if the ventricular rateremains within the fast or slow VT rate zone. In one embodiment, thetachyarrhythmia episode is considered sustaining when an averageventricular rate (such as an average of ventricular rates detectedwithin a moving window) falls within the fast or slow VT rate zone. Inanother embodiment, the tachyarrhythmia episode is considered sustainingwhen a predetermined majority of ventricular beats within a movingdetection window are fast beats, such as when at least six out of tenheart beats are fast beats. In one embodiment, if the SRD is programmedto be applied (“ON”), its value is programmable between 10 seconds and60 minutes, with approximately three minutes as a specific example. TheSRD is applied to determine whether a detected tachyarrhythmia needs tobe treated because of a sustaining high ventricular rate, after thetachyarrhythmia is classified to be a type that is not to be treated.Thus, the SRD functions as a “safety net” capable of overriding atachyarrhythmia classification to deliver a therapy. During the SRD,tachyarrhythmia classifier 346 continues to classify the detectedtachyarrhythmia and update the classification when necessary. If, forexample, the classification changes from SVT to VT during the SRD, aventricular anti-tachyarrhythmia therapy is to be delivered.

In one embodiment, the AV association and the confidence level are notused in selecting the ATP mode, such as in the method described withreference to FIG. 8. Thus, if ICD 210 does not use the AV associationand the confidence level for selecting the ATP mode or any otherpurpose, tachyarrhythmia detection and classification circuit 320 doesnot necessarily include confidence level analyzer 358 and AV associationdetector 360.

FIG. 4 is a flow chart illustrating a method 400 for classifying adetected tachyarrhythmia. In one embodiment, tachyarrhythmia classifier346 performs method 400. The atrial rate, ventricular rate, onset rate,stability parameter, correlation coefficient, and various thresholdsused in method 400 are detected, produced, or programmed as discussedwith reference to FIG. 3 above. For correlation analysis, the templatewaveform is produced using a cardiac signal sensed during an NSR.

A tachyarrhythmia is detected at 410, when the ventricular rate iswithin a predetermined tachyarrhythmia rate zone. If the ventricularrate (V-RATE) exceeds the atrial rate (A-RATE) by a predetermined marginat 412, the detected tachyarrhythmia is classified as VT. If theventricular rate does not exceed the atrial rate by a predeterminedmargin at 412, the atrial rate is compared to a predetermined thresholdatrial rate (A-RATE_(TH)) at 416.

If the atrial rate does not exceed the predetermined threshold atrialrate at 416, the onset rate indicates a gradual onset of tachyarrhythmiaat 418, and the correlation coefficient (FCC) exceeds a first marginalcorrelation threshold (FCC_(MTH1)) (i.e., FCC falls between FCC_(MTH1)and FCC_(TH)) at 418, the detected tachyarrhythmia is classified as ST.ST is a physiologic tachyarrhythmia originated in an SA node when the SAnode generates the electrical impulses at a tachyarrhythmic rate. In oneembodiment, the first marginal correlation coefficient is programmablebetween 0.4 and the correlation threshold (i.e.,0.4≦FCC_(MTH1)≦FCC_(TH)), with approximately 0.8 being a specificexample. In one embodiment, the first marginal correlation threshold isset to be lower than the correlation threshold by a predeterminedamount, such as approximately 0.2 (i.e., FCC_(MTH1)≈FCC_(TH)−0.2).

If the atrial rate exceeds a predetermined threshold atrial rate at 416,and the ventricular rate is unstable at 422, the detectedtachyarrhythmia is classified as AF. If the ventricular rate is stableat 422, the atrial rate exceeds the ventricular rate by a predeterminedmargin, and the correlation coefficient exceeds a second marginalcorrelation threshold (FCC_(MTH2)) (i.e., FCC falls between FCC_(MTH2)and FCC_(TH)) at 424, the detected tachyarrhythmia is classified as AFL.In one embodiment, the second marginal correlation threshold isprogrammable between 0.4 and the correlation threshold (i.e.,0.4≦FCC_(MTH2)≦FCC_(TH)), with approximately 0.8 being a specificexample. In one embodiment, the second marginal correlation threshold isset to be lower than the correlation threshold by a predeterminedamount, such as approximately 0.2 (i.e., FCC_(MTH2)≈FCC_(TH)−0.2).

If the atrial rate approximately equals to the ventricular rate, theonset rate indicates a sudden onset of tachyarrhythmia, the atrial andventricular events occur in a specified SVT pattern, and the correlationcoefficient exceeds a third marginal correlation threshold (FCC_(MTH3))(i.e., FCC falls between FCC_(MTH3) and FCC_(TH)) at 420, the detectedtachyarrhythmia is classified as AT. In one embodiment, the atrial rateis considered to be approximately equal to the ventricular rate when thedifference between the two rates is below 10 bpm. The detection ofcardiac event patterns including the SVT pattern is discussed in U.S.patent application Ser. No. 11/276,213, entitled “RHYTHM DISCRIMINATIONOF SUDDEN ONSET AND ONE-TO-ONE TACHYARRIIYTHMIA”, filed on Feb. 17,2006, assigned to Cardiac Pacemakers, Inc., which is hereby incorporatedin its entirety. If these conditions are not met at 420, the correlationcoefficient is compared to the correlation threshold (FCC_(TH)) at 426.AT is a pathologic tachyarrhythmia that occurs when a biologic pacemaker(focus) in an atrium usurps control of the heart rate from the SA node.In one embodiment, the third marginal correlation threshold isprogrammable between 0.4 and the correlation threshold (i.e.,0.4≦FCC_(MTH3)≦FCC_(TH)), with approximately 0.6 being a specificexample. In one embodiment, the third marginal correlation threshold isset to be lower than the correlation threshold by a predeterminedamount, such as approximately 0.4 (i.e., FCC_(MTH3)≈FCC_(TH)−0.4).

If the correlation coefficient exceeds the correlation threshold at 426,the detected tachyarrhythmia is classified as SVT. If the correlationcoefficient does not exceed the correlation threshold at 426, thedetected tachyarrhythmia is classified as VT. In one embodiment, thecorrelation threshold is programmable between 0.6 and 0.99, withapproximately 0.94 being a specific example.

FIG. 5 is a block diagram illustrating an embodiment of a circuit of apacing controller 532, which represents a specific embodiment of pacingcontroller 232. Pacing controller 532 includes an ATP control circuit570 that controls delivery of atrial and/or ventricular pacing pulsesaccording to a selected ATP mode. ATP control circuit 570 includes anATP mode selector 572, an AV-ATP enabler 574, a pilot A-ATP controller576, a pilot A-ATP response detector 578, and an ATP timer 580.

ATP mode selector 572 selects one of the A-ATP mode, V-ATP mode, andconcurrent AV-ATP mode according to one or more specified ATP modeselection criteria. In one embodiment, ATP mode selector 572 selects oneof the A-ATP mode, V-ATP mode, and synchronized AV-ATP mode according toa set of ATP mode selection criteria. In one embodiment, ATP modeselector 572 selects the synchronized AV-ATP mode in response to adetection of the stable AV association. In various embodiments,following the detection of a tachyarrhythmia, ATP mode selector 572 alsoselects the synchronized AV-ATP mode when (i) the detectedtachyarrhythmia is classified as VT and the correlation coefficient(produced by correlation analyzer 354) exceeds a specified correlationthreshold indicative of a VT origin at ventricular base (high septum),(ii) the atrial rate approximately equals to the ventricular rate andthe correlation coefficient exceeds a specified marginal correlationthreshold indicative of an origin of tachyarrhythmia near an atrium,(iii) the atrial rate is substantially higher than the ventricular rate,the correlation coefficient exceeds a specified marginal correlationthreshold indicative of an origin of tachyarrhythmia near an atrium, anda substantially stable AV ratio is detected, and/or (iv) the detectedtachyarrhythmia is classified as VT and the confidence level (producedby confidence level analyzer 364) is below a specified threshold level.If the synchronized AV-ATP mode is not selected, ATP mode selector 572selects the A-ATP mode when the detected tachyarrhythmia is classifiedas SVT (AF, AFL, or AT) but not ST, and selects the V-ATP mode when thedetected tachyarrhythmia is classified as VT. In one embodiment, thecorrelation threshold indicative of a VT origin at ventricular base(high septum) is specified as a value approximately between 0.6 and0.94; the marginal correlation threshold indicative of an origin oftachyarrhythmia near an atrium is specified as a value approximatelybetween 0.6 and 0.8; the specified threshold level for the confidencelevel is a value approximately between 0.8 and 0.9; the atrial rate isconsidered to be substantially higher than the ventricular rate when theatrial rate exceeds the ventricular rate by at least a margin specifiedas a value approximately between 10 and 50 bpm; and the atrial rate isconsidered to be substantially equal to the ventricular rate when thedifference between the two rates is less than a margin specified as avalue approximately between 0 and 10 bpm.

In another embodiment, ATP mode selector 572 selects one of the A-ATPmode, V-ATP mode, synchronized AV-ATP mode, and independent AV-ATP modeaccording to another set of ATP mode selection criteria. When the atrialrate exceeds a threshold atrial rate, and the ventricular rate is belowa threshold ventricular rate, ATP mode selector 572 selects the A-ATPmode. When the atrial rate does not exceed the threshold atrial rate,and the ventricular rate exceeds the threshold ventricular rate, ATPmode selector 572 selects the V-ATP mode. When the atrial rate exceedsthe threshold atrial rate, the ventricular rate exceeds the thresholdventricular rate, and the ventricular rate is substantially higher thanthe atrial rate, ATP mode selector 572 selects the V-ATP mode. When theatrial rate exceeds the threshold atrial rate, the ventricular rateexceeds the threshold ventricular rate, and the atrial rateapproximately equals the ventricular rate, ATP mode selector 572 selects(i) the synchronized AV-ATP mode when the detected tachyarrhythmia has asudden onset and the correlation coefficient exceeds the specifiedmarginal correlation threshold indicative of an origin oftachyarrhythmia near an atrium, (ii) the synchronized AV-ATP mode whenthe detected tachyarrhythmia has a gradual onset and the detectedtachyarrhythmia is not classified as ST, (iii) the V-ATP mode when thedetected tachyarrhythmia has a sudden onset and the correlationcoefficient does not exceed the specified marginal correlation thresholdindicative of an origin of tachyarrhythmia near an atrium, and (iv)inhibition of ATP therapy when the detected tachyarrhythmia has agradual onset and the detected arrhythmia is classified as ST. When theatrial rate exceeds the threshold atrial rate, the ventricular rateexceeds the threshold ventricular rate, and the atrial rate issubstantially higher than the ventricular rate, ATP mode selector 572selects (i) the independent AV-ATP mode when the correlation coefficientexceeds the specified marginal correlation threshold indicative of anorigin of tachyarrhythmia near an atrium, and (ii) the V-ATP mode whenthe correlation coefficient does not exceed the specified marginalcorrelation threshold indicative of an origin of tachyarrhythmia near anatrium.

AV-ATP enabler 574 allows selection of (or enables pacing at) theconcurrent AV-ATP mode according to one or more specified proarrhythmiaprevention criteria. In one embodiment, AV-ATP enabler 574 allowsselection of (or enables pacing at) the concurrent AV-ATP mode when theventricular rate falls within a specified rate zone. In a specificembodiment, the rate zone has an upper rate limit programmable between110 and 180 bpm. AV-ATP enabler 574 disallows selection of (or inhibitspacing at) the concurrent AV-ATP mode when the tachyarrhythmia isclassified as sinus tachycardia (ST).

In the illustrated embodiment, pilot A-ATP pacing is delivered todetermine whether to inhibit pacing at the synchronized AV-ATP mode.Pilot A-ATP controller 576 controls delivery of a pilot train of A-ATPpulses after the one or more specified ATP mode selection criteria aremet. Pilot A-ATP response detector 578 detects a response to thedelivery of the pilot train of A-ATP pulses. AV-ATP enabler 574disallows selection of (or inhibits pacing at) the synchronized AV-ATPmode in response to a detected response satisfying one or more specifiedsynchronized AV-ATP mode inhibition criteria. In one embodiment, pilotA-ATP controller 576 stops the delivery of the pilot train of A-ATPpulses in response to the detected response satisfying one or morespecified synchronized AV-ATP mode inhibition criteria. In oneembodiment, AV-ATP enabler 574 disallows selection of (or inhibitspacing at) the synchronized AV-ATP mode in response to at least one ofthe detection of a response indicative of ST and the detection of aresponse indicative of accelerated atrial and/or ventricular rates. In aspecific embodiment, pilot A-ATP controller 576 controls the delivery ofthe A-ATP pulses using a pacing rate set at approximately 5 to 20%higher than the detected intrinsic atrial rate. Pilot A-ATP responsedetector 578 indicates a detection of ST if the ventricular rateapproximately tracks the atrial rate during the delivery of the pilotA-ATP pulses. In one embodiment, pilot A-ATP response detector 578considers the ventricular rate to be approximately tracking the atrialrate when the difference between the atrial rate and the ventricularrate is within 5 bpm.

ATP timer 580 times the delivery of the atrial and ventricular pacingpulses. For the capability of timing the delivery of the atrial andventricular pacing pulses according to the synchronized AV-ATP mode, ATPtimer 580 includes a master channel selector 582, a master channelcoupling interval (CI) timer 584, a synchronization offset timer 586,and a slave channel coupling interval (CI) timer 590.

During the synchronized AV-ATP mode, master channel selector 582 selectsa master AV-ATP channel using the atrial rate stability parameter andthe ventricular rate stability parameter produced by stability analyzer352. In one embodiment, master channel selector 582 selects the channelassociated with the highest rate stability as the master AV-ATP channel.The master AV-ATP channel and at least one slave AV-ATP channel areenabled for ATP pulse delivery after the master AV-ATP channel isselected. Master channel coupling interval timer 584 starts a masterchannel coupling interval (CI_(m)) in response to a detection ofintrinsic depolarization in the master AV-ATP channel after the masterAV-ATP channel is enabled, and times the a master channel couplinginterval. The coupling interval is a time interval between the lastdetected intrinsic depolarization before the delivery of a burst of ATPpulses and the first pacing pulse of the burst. Delivery of the ATPpulses through the master AV-ATP channel starts upon the expiration ofthe master channel coupling interval. Synchronization offset timer 586starts a synchronization offset (ΔT_(m-s)) upon the expiration of themaster channel coupling interval, and times the synchronization offset.Slave channel coupling interval timer 590 starts a slave channelcoupling interval (CI_(s)) in response to a detection of intrinsicdepolarization in the slave AV-ATP channel after the ATP therapy in thesynchronized AV-ATP mode is initiated, and times the slave channelcoupling interval. Delivery of a burst of ATP pulses through the slaveAV-ATP channel starts upon the expiration of the slave channel couplinginterval or the expiration of the synchronization offset, whicheveroccurs later. This is to prevent the pacing pulse delivered through theslave AV-ATP channel from falling into the vulnerable period. In oneembodiment, synchronization offset timer 586 calculates thesynchronization offset as a fraction of an AV interval measured duringthe detected tachyarrhythmia when the stable AV association is detected,and calculates the synchronization offset as a fraction of an AVinterval measured during a normal sinus rhythm when the stable AVassociation is not detected. In one embodiment, slave channel couplinginterval timer 590 calculates the slave channel coupling interval as afunction of the master channel coupling interval when the rate stabilityparameter of the slave AV-ATP channel indicates a low stability, andcalculates the slave channel coupling interval as a function of themeasured heart rates of the slave channel when the rate stabilityparameter of the slave channel indicates a high stability. In oneembodiment, the rate stability parameter of each channel is the standarddeviation of the cardiac cycle length detected in that channel over aperiod of time. A high stability is indicated for a channel when thestandard deviation computed for that channel does not exceed a stabilitythreshold. A low stability is indicated for a channel when the standarddeviation computed for that channel exceeds the stability threshold. Inone embodiment, the stability threshold is 20 milliseconds.

During the independent AV-ATP mode, ATP timer 580 times the delivery ofthe atrial pacing pulses and the delivery of the ventricular pacingpulses individually and independently. In one embodiment, theindependent AV-ATP mode is substantially equivalent to the A-ATP modeand the V-ATP mode operating concurrently with independently determinedand timed atrial ATP parameters and ventricular ATP parameters. Afterthe independent AV-ATP mode is selected and the ATP therapy isinitiated, ATP timer 580 starts an atrial coupling interval (CIA) inresponse to a detection of intrinsic atrial depolarization and aventricular coupling interval (CI_(V)) in response to a detection ofintrinsic ventricular depolarization. The atrial coupling interval isdetermined using sensed intrinsic atrial rate. The ventricular couplinginterval is determined using sensed intrinsic ventricular rate. Deliveryof the atrial pacing pulses starts upon the expiration of the atrialcoupling interval. Delivery of the ventricular pacing pulses starts uponthe expiration of the ventricular coupling interval.

FIG. 6 is a flow chart illustrating an embodiment of a method 600 forcontrolling ATP. In one embodiment, method 600 is performed by ATPcontrol circuit 570.

At 610, cardiac signals indicative of atrial and ventriculardepolarizations are sensed. At 612, an atrial rate and a ventricularrate are detected using the sensed cardiac signals. At 614,tachyarrhythmia is detected using the ventricular rate and one or moretachyarrhythmia threshold rates. At 618, if a tachyarrhythmia episode isdetected at 616, the detected tachyarrhythmia is classified. At 622, ifit is determined at 620 that an ATP mode needs to be selected, the ATPmode is selected. In one embodiment, an ATP mode needs to be selected ifthe ATP therapy is programmed (enabled) and the ventricular rate isbelow a VF threshold rate. The detected tachyarrhythmia is classified asVF if the ventricular rate exceeds the VF threshold rate. The ATP modeis selected from available modes including the A-ATP mode, V-ATP mode,and concurrent AV-ATP mode using one or more specified ATP modeselection criteria and characteristics of the detected tachyarrhythmiaincluding the classification of the detected tachyarrhythmia. Theselection of the ATP mode is further discussed below with reference toFIGS. 7 and 8. At 624, atrial and/or ventricular pacing pulses aredelivered according to the selected ATP mode. The control of the ATPdelivery is further discussed below with reference to FIG. 9. At 626, ifit is determined at 620 that no ATP mode needs to be selected, anothertype of tachyarrhythmia therapy, such as a cardioversion/defibrillationtherapy, is selected using the classification of the detectedtachyarrhythmia and delivered to terminate the detected tachyarrhythmia.

FIG. 7 is a flow chart illustrating an embodiment of a method 700 forselecting an ATP mode. In one embodiment, method 700 is performed byportions of ATP control circuit 570, including at least ATP modeselector 572 and AV-ATP enabler 574.

At 710, selection of an ATP mode is started. At 712, a stable AVassociation is being detected using the atrial and ventriculardepolarizations sensed over a time interval. The stable AV associationis indicative of a substantially stable temporal relationship betweenthe atrial and ventricular depolarizations sensed over the timeinterval. In one embodiment, the stable AV association is detected whena substantially stable AV ratio is detected. The substantially stable AVratio is a substantially stable ratio of atrial depolarizations toventricular depolarizations. In another embodiment, the stable AVassociation is detected when substantially stable AV or VA intervals aredetected. The stable AV association is an indication for ATP therapy atthe synchronized AV-ATP mode. If the stable AV association is detectedat 712, the synchronized AV-ATP mode becomes the candidate modesubjected to at least another selection criterion at 722.

At 716, the A-ATP mode is selected when the stable AV association is notdetected at 712, and the detected tachyarrhythmia is classified as SVTat 714. If the detected tachyarrhythmia is not classified as SVT at 714,it is classified as VT. At 720, the V-ATP mode is selected when thestable AV association is not detected at 712, the detectedtachyarrhythmia is not classified as SVT at 714, and no other indicationfor ATP therapy at the synchronized AV-ATP mode is detected at 718. Ifthere is at least one other indication for ATP therapy at thesynchronized AV-ATP mode is detected at 718, the synchronized AV-ATPmode becomes the candidate mode subjected to at least another selectioncriterion at 722.

In various embodiments, examples for the other indications for ATPtherapy at the synchronized AV-ATP mode being detected at 718 include(i) a correlation coefficient exceeds a specified correlation thresholdindicative of a VT origin at ventricular base (high septum), (ii) theatrial rate approximately equals to the ventricular rate, and thecorrelation coefficient produced exceeds a specified marginalcorrelation threshold indicative of an origin of tachyarrhythmia near anatrium, (iii) the atrial rate is substantially higher than theventricular rate, the correlation coefficient exceeds a specifiedmarginal correlation threshold indicative of an origin oftachyarrhythmia near an atrium, and a substantially stable AV ratio isdetected, and (iv) the detected tachyarrhythmia is classified as VT andthe confidence level is below a specified threshold level. Thecorrelation coefficient represents the correlation between a waveform ofthe cardiac signal sensed during the detected tachyarrhythmia and atemplate waveform of the cardiac signal sensed during a normal sinusrhythm, such as the correlation coefficient produced by correlationanalyzer 354. The confidence level represents the level of confidence ina correct VT classification, such as the confidence level produced byconfidence level analyzer 364. These indications indicate an origin oftachyarrhythmia in atria, atrio-ventricular node, or ventricular basal(high-septal) areas or a reentrant loop that covers at least an atriumand a ventricle.

At 724, if the detected arrhythmia is classified as ST at 722, the ATPtherapy is inhibited. In one embodiment, a pilot train of A-ATP pulsesis delivered in response to the synchronized AV-ATP mode becoming thecandidate mode. The response to the delivery of the pilot train of A-ATPpulses is detected. The ATP therapy is inhibited at 724 if the detectedresponse does not satisfy one or more specified synchronized AV-ATP modeinhibition criteria. In one embodiment, the ATP therapy is inhibited at724 in response to the detection of a response indicative of ST or inresponse to the detection of a response indicative of accelerated atrialor ventricular rate. At 726, if the detected arrhythmia is notclassified as ST at 732, the synchronized AV-ATP mode is selected. At728, the selection of ATP mode concludes with the ATP mode selected orthe ATP therapy inhibited.

FIG. 8 is a flow chart illustrating an embodiment of a method 800 forselecting an ATP mode. In one embodiment, method 800 is performed byportions of ATP control circuit 570, including at least ATP modeselector 572 and AV-ATP enabler 574. To perform method 800, ATP modeselector 572 need not receive inputs from confidence level analyzer 358and AV association detector 360. Consequently, confidence level analyzer358 and AV association detector 360 are not required in system 100 ifonly method 800 is performed for ATP mode selection.

At 810, selection of an ATP mode is started. At 816, if the ventricularrate is below a threshold ventricular rate (V-RATE_(TH)) at 812, and theatrial rate exceeds a threshold atrial rate (A-RATE_(TH)) at 814, andthe A-ATP mode is selected. If the atrial rate does not exceed thethreshold atrial rate at 814, the ATP therapy is inhibited at 830.

If the ventricular rate is not below the threshold ventricular rate at812, the atrial rate and the ventricular rate is compared. At 820,whether the ventricular rate is substantially higher than the atrialrate is determined. If the ventricular rate is substantially higher thanthe atrial rate at 820, the V-ATP mode is selected at 818. At 822, it isdetermined whether the atrial rate approximately equals to theventricular rate. If the atrial rate approximately equals theventricular rate, it is determined whether the detected tachyarrhythmiahas a sudden onset, whether the correlation coefficient exceeds aspecified marginal correlation threshold indicative of an origin of thedetected tachyarrhythmia near an atrium, and whether the detectedarrhythmia is an ST. If the detected tachyarrhythmia has a sudden onsetat 824, and the correlation coefficient exceeds the specified marginalcorrelation threshold at 826, the synchronized AV-ATP mode is selectedat 836. If the detected tachyarrhythmia has a gradual onset at 824, andthe detected arrhythmia is classified as ST at 828, the ATP therapy isinhibited at 830. If the detected tachyarrhythmia has a gradual onset at824, and the detected arrhythmia is not classified as ST at 828, thesynchronized AV-ATP mode is selected at 836. If the detectedtachyarrhythmia has a sudden onset at 824, and the correlationcoefficient does not exceed the specified marginal correlation thresholdat 826, the V-ATP mode is selected at 818.

If the atrial rate does not approximately equal to the ventricular rate(i.e., if the atrial rate is substantially higher than the ventricularrate), it is determined whether the correlation coefficient exceeds thespecified marginal correlation threshold. If the correlation coefficientexceeds the specified marginal correlation threshold at 832, theindependent AV-ATP mode is selected at 836. If the correlationcoefficient does not exceed the specified marginal correlation thresholdat 832, the V-ATP mode is selected at 818. At 838, the selection of ATPmode concludes with the ATP mode selected or the ATP therapy inhibited.

FIG. 9 is a flow chart illustrating an embodiment of a method 900 fortiming ATP delivery according to the synchronized AV-ATP mode. In oneembodiment, method 900 is performed by ATP timer 580.

At 910, delivery of an ATP therapy in the synchronized AV-ATP mode isstarted. At 912, a master AV-ATP channel is selected using the atrialrate stability parameter indicative of the stability of the atrial rateand the ventricular rate stability parameter indicative of the stabilityof the ventricular rate. The channel associated with the highest ratestability is selected as the master AV-ATP channel. The master AV-ATPchannel and at least one slave AV-ATP channel are enabled after themaster AV-ATP channel is selected.

At 914, a master channel coupling interval (CI_(m)) is timed after themaster AV-ATP channel is enabled. The master channel coupling intervalstarts with an intrinsic depolarization detected through the masterAV-ATP channel. At 916, a burst of ATP pulses are delivered through themaster AV-ATP channel, with the leading pulse of the burst deliveredupon the expiration of the master channel coupling interval.

At 918, a synchronization offset (ΔT_(m-s)) is timed. Thesynchronization offset starts upon the expiration of master channelcoupling interval. In one embodiment, the synchronization offset iscalculated as a fraction of an AV interval measured during the detectedtachyarrhythmia when the stable AV association is detected by AVassociation detector 360, and is calculated as a fraction of an AVinterval measured during a normal sinus rhythm when the stable AVassociation is not detected.

At 920, a slave channel coupling interval (CI_(s)) is timed after theslave AV-ATP channel is enabled. The slave channel coupling intervalstarts with an intrinsic depolarization detected through the slaveAV-ATP channel after the slave AV-ATP channel is enabled. In oneembodiment, the slave channel coupling interval is calculated as afunction of the master channel coupling interval when the rate stabilityparameter of the slave AV-ATP channel indicates a low stability, and iscalculated as a function of the measured heart rates of the slavechannel when the rate stability parameter of the slave channel indicatesa high stability.

At 922, a burst of ATP pulses are delivered through the slave AV-ATPchannel, with the leading pulse of the burst delivered upon theexpiration of the slave channel coupling interval or the expiration ofthe synchronization offset, whichever occurs later. At 924, the deliveryof the ATP therapy is concluded.

In one embodiment, the master AV-ATP channel is selected from an atrialchannel for delivering the atrial pacing pulses and a ventricularchannel for delivering the ventricular pacing pulses. In variousembodiments, the master AV-ATP channel is selected as the channelassociated with the highest rate stability, among two or more channelseach allowing delivery of pacing pulses to a specified cardiac location.

When the stable AV association is detected, using the synchronizationoffset improves capture for both the master and slave AV-ATP channels bymaking use of the substantially stable association between thedepolarization in these channels.

When the master AV-ATP channel is the channel through which atrialpacing pulses are delivered, using the synchronization offset temporallyaligns the delivery of the atrial pacing pulses with the delivery of theventricular pacing pulses, thereby avoiding delivery the atrial pacingpulses into the vulnerable period.

It is to be understood that the above detailed description is intendedto be illustrative, and not restrictive. Other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

1. An implantable cardiac rhythm management device, comprising: a pacingcircuit configured to deliver atrial pacing pulses and ventricularpacing pulses; a tachyarrhythmia detection and classification circuitincluding: a cardiac sensing circuit configured to sense cardiac signalsindicative of atrial depolarizations and ventricular depolarizations; arate detector configured to detect an atrial rate and a ventricular rateusing the cardiac signals; a tachyarrhythmia detector configured todetect tachyarrhythmia using the ventricular rate and one or moretachyarrhythmia threshold rates; and a tachyarrhythmia classifierconfigured to classify the detected tachyarrhythmia; and ananti-tachycardia pacing (ATP) control circuit coupled to the pacingcircuit and the tachyarrhythmia detection and classification circuit,the ATP control circuit configured to control delivery of the atrialpacing pulses and the ventricular pacing pulses according to a pluralityof ATP modes including an atrial ATP (A-ATP) mode, a ventricular ATP(V-ATP) mode, and a concurrent atrio-ventricular ATP (concurrent AV-ATP)mode, the concurrent ATP mode being an ATP mode during which the atrialpacing pulses and the ventricular pacing pulses are deliveredconcurrently, the ATP control circuit including an ATP mode selectorconfigured to select an ATP mode from the plurality of ATP modes usingone or more specified ATP mode selection criteria and the classificationof the detected tachyarrhythmia.
 2. The device of claim 1, wherein theconcurrent AV-ATP mode comprises a synchronized atrio-ventricular ATP(synchronized AV-ATP) mode during which the atrial pacing pulses and theventricular pacing pulses are delivered synchronously, and the ATP modeselector is configured to select one of the A-ATP mode, the V-ATP mode,and the synchronized AV-ATP mode using the one or more specified ATPmode selection criteria and the classification of the detectedtachyarrhythmia.
 3. The device of claim 2, comprising anatrio-ventricular (AV) association detector configured to detect astable AV association using the atrial depolarizations and ventriculardepolarizations sensed over a time interval, the stable AV associationindicative of a substantially stable temporal relationship between theatrial depolarizations and ventricular depolarizations sensed over thetime interval, and wherein the ATP mode selector is configured to selectthe synchronized AV-ATP mode in response to a detection of the stable AVassociation.
 4. The device of claim 3, wherein the AV associationdetector comprises one or more of: a stable AV ratio detector configuredto detect a substantially stable AV ratio being a substantially stableratio of atrial depolarizations to ventricular depolarizations andindicate the detection of the stable AV association when thesubstantially stable AV ratio is detected; and a stable intervaldetector configured to detect substantially stable AV orventriculo-atrial (VA) intervals and indicate the detection of thestable AV association when the substantially stable AV or VA intervalsare detected.
 5. The device of claim 3, wherein the tachyarrhythmiaclassifier comprises a correlation analyzer configured to produce acorrelation coefficient representative of a correlation between atachyarrhythmic waveform sensed during the detected tachyarrhythmia anda template waveform, and the ATP mode selector is configured to selectthe synchronized AV-ATP mode when: the tachyarrhythmia is classified asVT and the correlation coefficient exceeds a specified correlationthreshold indicative of a VT origin at a ventricular base; the atrialrate approximately equals the ventricular rate and the correlationcoefficient exceeds a specified marginal correlation threshold; and theatrial rate is substantially higher than the ventricular rate, thecorrelation coefficient exceeds a specified marginal correlationthreshold, and the substantially stable AV ratio is detected.
 6. Thedevice of claim 2, comprising a confidence level analyzer configured todetermine a confidence level associated with a VT classification of thedetected tachyarrhythmia, wherein the ATP mode selector is configured toselect the synchronized AV-ATP mode in response to the VT classificationof the tachyarrhythmia and a confidence level below a specifiedthreshold level, the confidence level associated with the VTclassification.
 7. The device of claim 2, comprising an AV-ATP enablerconfigured to allow selection of the synchronized AV-ATP mode accordingto one or more specified proarrhythmia prevention criteria.
 8. Thedevice of claim 7, wherein the AV-ATP enabler comprises: a pilot A-ATPcontroller configured to deliver a pilot train of A-ATP pulses inresponse to a selection of the synchronized AV-ATP mode; and a pilotA-ATP response detector configured to detect a response to the deliveryof the pilot train of A-ATP pulses, wherein AV-ATP enabler to disallowselection of the synchronized AV-ATP mode in response to at least one ofa detected response indicative of ST and a detected response indicativeof accelerated atrial or ventricular rate.
 9. The device of claim 2,wherein the tachyarrhythmia classifier comprises a stability analyzer toproduce at least an atrial rate stability parameter indicative of astability of the atrial rate and a ventricular rate stability parameterindicative of a stability of the ventricular rate, and the ATP controlcircuit comprises an ATP timer configured to time the delivery of theatrial and ventricular pacing pulses according to the selected ATP mode,the ATP timer including: a master channel selector configured to selecta master AV-ATP channel using the atrial rate stability parameter andthe ventricular rate stability parameter; a master channel couplinginterval timer configured to time a master channel coupling interval forthe master AV-ATP channel after the master AV-ATP channel is selected; asynchronization offset timer configured to start a synchronizationoffset upon expiration of the master channel coupling interval; and aslave channel coupling interval timer configured to time a slave channelcoupling interval for a slave AV-ATP channel after the master AV-ATPchannel is selected, wherein the ATP control circuit is configured tostart delivery of the pacing pulses through the slave AV-ATP channelupon expiration of the slave channel coupling interval or expiration ofthe synchronization offset, whichever occurs later.
 10. The device ofclaim 1, wherein the concurrent AV-ATP mode comprises: a synchronizedatrio-ventricular ATP (synchronized AV-ATP) mode during which the atrialpacing pulses and the ventricular pacing pulses are deliveredsynchronously; and an independent atrio-ventricular ATP (independentAV-ATP) mode during which delivery of the atrial pacing pulses anddelivery of the ventricular pacing pulses are timed independently, andwherein the ATP mode selector is configured to select one of the A-ATPmode, the V-ATP mode, the synchronized AV-ATP mode, and the independentAV-ATP mode using the one or more specified ATP mode selection criteriaand the classification of the detected tachyarrhythmia.
 11. The deviceof claim 10, wherein the tachyarrhythmia classifier comprises: acorrelation analyzer configured to produce a correlation coefficientrepresentative of a correlation between a tachyarrhythmic waveformsensed during the detected tachyarrhythmia and a template waveform; andan onset rate analyzer configured to determine whether the detectedtachyarrhythmia has a gradual onset indicative of physiologicaltachyarrhythmia or a sudden onset indicative of pathologicaltachyarrhythmia, and wherein the ATP mode selector is configured toselect one of the A-ATP mode, V-ATP mode, synchronized AV-ATP mode, andindependent AV-ATP mode using the correlation coefficient and whetherthe detected tachyarrhythmia has the gradual onset or the sudden onset.12. The device of claim 11, wherein the ATP mode selector is configuredto select, when the atrial rate exceeds the threshold atrial rate, theventricular rate exceeds the threshold ventricular rate, and the atrialrate approximately equals the ventricular rate: the synchronized AV-ATPmode when the detected tachyarrhythmia has the sudden onset and thecorrelation coefficient exceeds a specified marginal correlationthreshold; the synchronized AV-ATP mode when the detectedtachyarrhythmia has the gradual onset and the detected tachyarrhythmiais not classified as ST; the V-ATP mode if the detected tachyarrhythmiahas the sudden onset and the correlation coefficient does not exceed thespecified marginal correlation threshold; and inhibition of ATP therapywhen the detected tachyarrhythmia has a gradual onset and the detectedarrhythmia is classified as ST.
 13. The device of claim 11, wherein theATP mode selector is configured to select, when the atrial rate exceedsthe threshold atrial rate, the ventricular rate exceeds the thresholdventricular rate, and the atrial rate is substantially higher than theventricular rate: the independent AV-ATP mode when the correlationcoefficient exceeds a specified marginal correlation threshold; and theV-ATP mode when the correlation coefficient does not exceed thespecified marginal correlation threshold.
 14. A method for operating animplantable cardiac rhythm management device, the method comprising:providing a plurality of anti-tachycardia pacing (ATP) modes forcontrolling delivery of atrial pacing pulses and ventricular pacingpulses, the ATP modes including an atrial ATP (A-ATP) mode, aventricular ATP (V-ATP) mode, and a concurrent atrio-ventricular ATP(concurrent AV-ATP) mode, the concurrent ATP mode being an ATP modeduring which the atrial pacing pulses and the ventricular pacing pulsesare delivered concurrently; sensing cardiac signals indicative of atrialdepolarizations and ventricular depolarizations; detecting an atrialrate and a ventricular rate using the cardiac signals; detectingtachyarrhythmia using the ventricular rate and one or moretachyarrhythmia threshold rates; classifying the detectedtachyarrhythmia; selecting an ATP mode from the plurality of ATP modesusing one or more specified ATP mode selection criteria and theclassification of the detected tachyarrhythmia; and controlling thedelivery of the atrial pacing pulses and the ventricular pacing pulsesaccording to the selected ATP mode.
 15. The method of claim 14, whereinthe concurrent AV-ATP mode comprises a synchronized atrio-ventricularATP (synchronized AV-ATP) mode during which the atrial pacing pulses andthe ventricular pacing pulses are delivered synchronously, and selectingthe ATP mode comprises selecting one of the A-ATP mode, the V-ATP mode,and the synchronized AV-ATP mode using the one or more specified ATPmode selection criteria and the classification of the detectedtachyarrhythmia.
 16. The method of claim 15, comprising detecting astable atrio-ventricular (AV) association using the atrialdepolarizations and ventricular depolarizations sensed over a timeinterval, the stable AV association indicative of a substantially stabletemporal relationship between the atrial depolarizations and ventriculardepolarizations sensed over the time interval, and wherein selecting theATP mode comprises selecting the synchronized AV-ATP mode in response toa detection of the stable AV association.
 17. The method of claim 16,comprising producing a correlation coefficient representative of acorrelation between a tachyarrhythmic waveform sensed during thedetected tachyarrhythmia and a template waveform, and wherein selectingthe ATP mode comprising: selecting the synchronized AV-ATP mode inresponse to a VT classification of the tachyarrhythmia and thecorrelation coefficient exceeding a specified correlation thresholdindicative of a VT origin at a ventricular base; selecting thesynchronized AV-ATP mode in response to an atrial rate approximatelyequaling the ventricular rate and the correlation coefficient exceedinga specified first marginal correlation threshold; and selecting thesynchronized AV-ATP mode in response to the atrial rate substantiallyhigher than the ventricular rate, the correlation coefficient exceedinga specified marginal correlation threshold, and the substantially stableAV ratio.
 18. The method of claim 15, comprising: allowing selection ofthe synchronized AV-ATP mode when the ventricular rate falls within aspecified rate zone; and disallowing selection of the synchronizedAV-ATP mode when the tachyarrhythmia is classified as sinus tachycardia(ST).
 19. The method of claim 15, comprising: delivering a pilot trainof A-ATP pulses in response to a selection of the synchronized AV-ATPmode; detecting a response to the delivery of the pilot train of A-ATPpulses; and disallowing selection of the synchronized AV-ATP mode inresponse to at least one of the detection of a response indicative of STand the detection of a response indicative of accelerated atrial orventricular rate.
 20. The method of claim 15, comprising: producing atleast an atrial rate stability parameter indicative of a stability ofthe atrial rate and a ventricular rate stability parameter indicative ofa stability of the ventricular rate; selecting a master AV-ATP channelusing the atrial rate stability parameter and the ventricular ratestability parameter; timing a master channel coupling interval after themaster AV-ATP channel is selected; starting delivery of pacing pulsesthrough the master AV-ATP channel upon expiration of the master channelcoupling interval; starting a synchronization offset upon the expirationof the master channel coupling interval; timing a slave channel couplinginterval after the master AV-ATP channel is selected; starting deliveryof pacing pulses through the slave AV-ATP channel upon expiration of theslave channel coupling interval or expiration of the synchronizationoffset, whichever occurs later.
 21. The method of claim 14, wherein theconcurrent AV-ATP mode comprises: a synchronized atrio-ventricular ATP(synchronized AV-ATP) mode during which the atrial pacing pulses and theventricular pacing pulses are delivered synchronously; and anindependent atrio-ventricular ATP (independent AV-ATP) mode during whichdelivery of the atrial pacing pulses and delivery of the ventricularpacing pulses are timed independently, and wherein selecting the ATPmode comprises selecting one of the A-ATP mode, the V-ATP mode, thesynchronized AV-ATP mode, and the independent AV-ATP mode using the oneor more specified ATP mode selection criteria and the classification ofthe detected tachyarrhythmia.
 22. The method of claim 21 whereinselecting the ATP mode comprises selecting the independent AV-ATP modewhen timing of the delivery of the atrial pacing pulses and theventricular pacing pulses according to the synchronized AV-ATP mode isinfeasible.
 23. The method of claim 21, comprising: producing acorrelation coefficient representative of a correlation between atachyarrhythmic waveform sensed during the detected tachyarrhythmia anda template waveform; and determining whether the detectedtachyarrhythmia has a gradual onset indicative of physiologicaltachyarrhythmia or a sudden onset indicative of pathologicaltachyarrhythmia, and wherein selecting the ATP mode comprises selectingone of the A-ATP mode, V-ATP mode, synchronized AV-ATP mode, andindependent AV-ATP mode using the correlation coefficient and whetherthe detected tachyarrhythmia has the gradual onset or the sudden onset.24. The method of claim 23, wherein selecting the ATP mode comprisesselecting, when the atrial rate exceeds the threshold atrial rate, theventricular rate exceeds the threshold ventricular rate, and the atrialrate approximately equals the ventricular rate: the synchronized AV-ATPmode when the detected tachyarrhythmia has the sudden onset and thecorrelation coefficient exceeds a specified marginal correlationthreshold; the synchronized AV-ATP mode when the detectedtachyarrhythmia has the gradual onset and the detected tachyarrhythmiais not classified as ST; the V-ATP mode if the detected tachyarrhythmiahas the sudden onset and the correlation coefficient does not exceed thespecified marginal correlation threshold; and inhibition of ATP therapywhen the detected tachyarrhythmia has a gradual onset and the detectedarrhythmia is classified as ST.
 25. The method of claim 23, whereinselecting the ATP mode comprises selecting, when the atrial rate exceedsthe threshold atrial rate, the ventricular rate exceeds the thresholdventricular rate, and the atrial rate is substantially higher than theventricular rate: the independent AV-ATP mode when the correlationcoefficient exceeds a specified marginal correlation threshold; and theV-ATP mode when the correlation coefficient does not exceed thespecified marginal correlation threshold.